The invention relates to superconductors in general and more particularly to a current feed for a superconducting magnet coil which is cooled by a cryogenic medium.
In a superconducting magnet coil, the coil ends can be short circuited by a continuous current switch. This switch has a disconnecting device at each coil end which contains a stationary contact member which is connected to the respective coil end and is included in the cooling effect of a cryogenic medium cooling the coil, and a movable contact member connected to a current supply, as well as a mechanical actuating device for joining the contact members with a predetermined contact pressure and for separating them after the magnet coil is short-circuited.
For feeding current into magnet coils with deep cooled superconductors, current feeding devices are required, through which an electric current is fed to these conductors from a power supply which is at a higher temperature level, e.g., at room temperature. The conductors of the magnet coil are held at a temperature level below the so-called transition temperature of its superconductive material by means of a cryogenic medium, for instance, liquid helium. Since this transition temperature of the known superconductive materials is far below room temperature, conductor parts of electrically normally conducting material such as copper or aluminum are used for bridging the temperature differences in the current feeds. These normally conducting conductor parts are then connected to the superconductors of the magnet coil at a point which is also held at a temperature level below the transition temperature of the superconductor material.
If a magnetic field has been set up in such a magnet coil by a corresponding current, the coil can be short-circuited via a continuous-current switch, because practically no energy need be supplied to it any more from the outside to maintain the field. Only the energy required to maintain the superconducting state of the coil must then still be supplied. Switches with a particularly low resistance are suited as continuous current switches, so that current can flow in the shorted circuit formed by the coil and the continuous current switch almost without attenuation. (see, for instance, U.S. Pat. No. 4,024,363 and German Offenlegungsschrift No. 27 07 589).
However, heat is always fed to the cryogenic medium which cools the conductors of the magnet coil through the normally conducting conductor parts of a current feed, which are still connected, although current need no longer be supplied, during the continuous operation when the magnet coil is excited and shorted. To avoid corresponding heat losses, the current feed can therefore be provided with an interrupting device, in order to disconnect electrically and thermally highly conducting conductor parts of the current feed which are connected to the power supply, which is at room temperature, during continuous operation of the magnet coil, from conductor parts which are in the cryogenic medium (see, for instance, the journal "Elektrie" vol. 19 (1965), no. 4, page 179). A suitable disconnecting device contains, in general, a stationary cold contact element and movable warm contact element as well as a mechanical actuating device, with which the contact elements can be joined together with a predetermined contact pressure and can be disconnected from each other after the magnet coil is shorted.
In the design of such a current feed with a disconnecting device, the difficulty arises of necessity that the remote contact element which is warmed up to, say, room temperature, must be brought into thermal contact with the cold contact element and therefore warms up the latter. The greatly different specific heats of these two contact elements must be taken into consideration in this connection. Then, the danger may exist that too much heat will be introduced into the cold contact element and thereby into the coil lead connected thereto and the superconductive material will become normally conducting at least at that point. This danger is present mainly prior to a de-energizing action, when the full current still flows in the magnet coil.